1. Field of the Invention
The present invention relates to a printing position adjusting method for dots printed on a print medium and to a corresponding printing system, host apparatus and program.
2. Description of the Related Art
In recent years, relatively inexpensive office equipment including personal computers, word processors and the like have proliferated. Consequently, various printing apparatuses that print information inputted via such equipment as well as techniques that enable the apparatuses to operate at high speed or with high quality are being developed at a rapid pace. Among such printing apparatuses, a serial printer using a dot matrix printing method has been attracting attention as a printing apparatus (printer) capable of realizing high speed or high quality printing at low cost.
In the case of a printing apparatus that performs, for example, bidirectional printing to achieve high speed, misalignment of positions of dots formed in a forward scan and positions of dots formed in a backward scan on a print medium causes ruled line misalignment and therefore a degradation in print quality. That is, when vertical ruled lines perpendicular to the scan direction of the print head are alternately formed in forward scans and backward scans, the positions of dots printed in the forward scans may fail to align with those printed in the backward scans, causing the ruled lines to lose their straightness. This line misalignment is one of the most common forms of print quality degradation perceived by users. Since ruled lines are often printed in black, line misalignment tends to be perceived as a problem encountered in black images. However, similar phenomena occur with images in which ruled lines are formed in other colors.
Such a misalignment between the positions of dots printed during forward scans and backward scans may have an adverse effect on an image, causing a phenomenon called “texture” when multi-pass printing is performed in order to enhance print quality. Multi-pass printing refers to a print method in which image data corresponding to a predetermined area on a print medium is divided among a plurality of print scans using a mask pattern, whereby the predetermined area is completed by a plurality of print scans. When using multi-pass printing, although phenomena such as the aforementioned ruled line misalignment are unlikely to be perceived even when misalignments occur between the positions of dots printed during forward scans and backward scans, there are cases where an unpleasant pattern (texture) is perceived in an image. Such a texture appears in periods dependent on the applied mask pattern, and tends to become particularly noticeable in half tone areas of a printed image having a high density and a high contrast, such as when printing is performed in monochrome or on coated paper.
Further, in the case of a printing apparatus having a plurality of print heads such as four print heads that respectively print the four colors of yellow, magenta, cyan and black, if a misalignment occurs among the printing positions of the four print heads, a phenomenon called “color misalignment” occurs on the image.
The “color misalignment” phenomenon will now be described briefly. A blue color (for example) is formed when a dot of magenta ink and a dot of cyan ink are printed at a predetermined position on a print medium. In this case, a slight color difference will occur between an area where dots of the two colors overlap and an area where such overlapping does not occur. In a uniform blue image, if an area with such a slightly different color exists, the area will not stand out in the image provided that the area is small. However, when a misalignment occurs between positions (printing positions) of dots where magenta and cyan are printed in a specific print scan, there will be a recognizable difference between the blue color in the area printed in that scan and the blue colors in other areas. This will result in a band-like non-uniform blue image. In the present specification, such a phenomenon shall be referred to as “color misalignment”. “Color misalignments” tend to be inconspicuous on plain paper, but become more noticeable on print mediums with higher color saturation such as coated paper.
When printing is performed at adjacent pixels by different print heads, if a misalignment occurs among the printing positions of dots printed by the respective print heads, gaps will form between the dots, thereby allowing the color of the print medium to be directly perceived. Since print mediums are mostly white, this phenomenon is referred to as an “unprinted portion”. This phenomenon is particularly noticeable with images having strong contrasts. For example, when a black image is formed and a white area exists in the image where dots are not printed, “unprinted portions” are more easily recognized due to the strong contrast between white and black.
For the purpose of suppressing such print quality degradation as described above, many printing apparatuses on the market adopt dot adjustment value acquisition processing. Dot adjustment value acquisition processing (also referred to as printing position adjustment) according to the present specification refers to a series of processes for adjusting the relative positional relationship between the printing position of a dot printed in a first printing operation and the printing position of a dot printed in a second printing operation. The dot adjustment value acquisition processing includes a process for acquiring an adjustment value for adjusting printing positions. In this case, the first printing operation and the second printing operation respectively refer to, for example, printing by a forward scan and a backward scan in bidirectional printing. In addition, the adjustment value acquired in dot adjustment value acquisition processing is, for example, a correction value for adjusting timings at which a print head discharges ink during forward and backward scans in order to adjust the relative positional relationship between the printing position of a dot printed in a forward scan and the printing position of a dot printed in a backward scan during bidirectional printing.
A general procedure for performing dot adjustment value acquisition processing will be described below using bidirectional printing as an example. First, the printing apparatus prints a test pattern for acquiring an adjustment value. When printing a test pattern, firstly, in a forward scan, the printing apparatus prints a plurality of straight lines (reference lines) oriented perpendicular to the scan direction at constant intervals. Next, without conveying the print medium, a backward scan is performed by the print head to print the same number of straight lines (shift lines) in correspondence to the straight lines printed in the forward scan. In the backward scan, a plurality of straight lines are printed while varying ink discharge timings so as to shift the relative positional relationships with the straight lines printed in the forward scan. In this manner, a test pattern is completed, in which a plurality of ruled line patterns (adjustment patterns) constituted by straight lines printed during a forward scan and those printed during a backward scan is produced.
A user then visually judges and selects a ruled line pattern that is either straight or is closest to a straight line among the plurality of outputted ruled line patterns. Subsequently, a parameter used when the selected ruled line pattern was formed is inputted either directly into the printing apparatus via key operations or the like or by operating a host apparatus connected to the printing apparatus. Based on the inputted parameter, the printing apparatus sets optimum discharge timings for adjusting printing positions of dots printed in a forward scan and in a backward scan. Thereafter, printing operations of the respective scans are performed according to the set discharge timings.
In the case where dot adjustment value acquisition processing is performed among a plurality of print heads, dot adjustment value acquisition processing can be performed in the same manner as in the example of bidirectional printing described above by, for example, having the plurality of print heads respectively print pluralities of straight lines oriented perpendicular to the scan direction.
The method heretofore described is a method in which a test pattern is printed to be visually judged by a user (hereinafter referred to as manual dot adjustment value acquisition processing). However, not only is this method troublesome for the user, there are also risks that judgmental and operational errors may occur. Accordingly, in recent years, a method of automatically performing dot adjustment value acquisition processing (hereinafter referred to as automatic dot adjustment value acquisition processing) through the use of an optical sensor has been proposed and put to practical use (for example, refer to Japanese Patent Laid-open No. 11-291470).
Specific processes carried out in the automatic dot adjustment value acquisition processing described in Japanese Patent Laid-open No. 11-291470 will now be briefly described using the case of bidirectional printing as an example. Similarly, with automatic dot adjustment value acquisition processing, a test pattern constituted by a plurality of adjustment patterns is first printed. When printing the test pattern, firstly, dots (reference dots) to be used as reference by the respective adjustment patterns are printed by a forward scan of the print head. Next, in a backward scan, for a plurality of adjustment patterns, dots (shift dots) are printed by shifting relative positions with respect to the reference dots by predetermined increments, thus completing the respective adjustment patterns.
The plurality of adjustment patterns is configured such that the mutual misalignment among the dots printed in the forward scan and dots printed in the backward scan result in a variance in the area factor of each adjustment pattern (in each adjustment pattern, the percentage of an area occupied by a dot with respect to the non-printed portion). The printing apparatus measures the respective average densities of the plurality of adjustment patterns using an optical sensor, whereby the pattern with the highest average density is judged to be the pattern having minimal printing position misalignment. Based on the adjustment pattern, the printing apparatus automatically sets an optimum discharge timing for adjusting printing positions with respect to each print scan by each print head. Such an automatic dot adjustment value acquisition processing eliminates the need for performing troublesome operations on the part of the user, and obviates the risks of judgmental and operational errors.
Nevertheless, if the configuration of a printing apparatus only allows printing position adjustment through automatic dot adjustment value acquisition processing, the occurrence of a situation during automatic dot adjustment value acquisition processing where normal operations cease due to an unforeseen cause makes printing position adjustment of dots impossible at that point. In this light, Japanese Patent Laid-open No. 11-291470 also discloses a configuration that accommodates both the automatic dot adjustment value acquisition processing and the manual dot adjustment value acquisition processing and, at the same time, prompts the user to perform the manual dot adjustment value acquisition processing only in the event that an error occurs during automatic dot adjustment value acquisition processing.
Furthermore, providing both manual dot adjustment value acquisition processing and automatic dot adjustment value acquisition processing enables dot adjustment value acquisition processing to be provided such that diversified needs of users ranging from those familiar with using printing apparatuses to novices can be accommodated.
With manual dot adjustment value acquisition processing, a user is required to perform operations for: having a printing apparatus print test patterns; observing the test patterns and selecting an optimum condition; and inputting the condition into the printing apparatus or the host apparatus. As seen, manual dot adjustment value acquisition processing requires that the user perform many troublesome procedures. Such tasks are particularly confusing and cumbersome to novice users who are not used to handling printing apparatuses. However, manual dot adjustment value acquisition processing wherein adjustment of printing positions is performed by visually confirming adjustment patterns through the user's own eyes enables users more experienced with the handling of printing apparatuses to perform adjustment in a satisfactory manner. Therefore, there may be cases where adjustment is performed with higher accuracy than automatic dot adjustment value acquisition processing.
On the other hand, automatic dot adjustment value acquisition processing wherein everything from printing test patterns to acquiring adjustment values is performed automatically is advantageous in that troublesome operations such as inputting on the part of the user are no longer necessary.
In other words, manual dot adjustment value acquisition processing is able to accommodate demands towards high accuracy printing position adjustment from experienced users. In addition, automatic dot adjustment value acquisition processing is able to accommodate demands towards adjusting printing positions without having to perform troublesome operations from novice users unfamiliar with the handling of printing apparatuses. Therefore, providing both manual dot adjustment value acquisition processing and automatic dot adjustment value acquisition processing enables dot adjustment value acquisition processing to be provided such that demands from both users familiar with using printing apparatuses and novice users can be accommodated.
However, in conventional dot adjustment value acquisition processing, inexpensive plain paper is generally used as a print medium. In other words, printing position adjustment is performed using plain paper for both processing that requires high accuracy printing position adjustment (e.g., manual dot adjustment value acquisition processing) and processing that does not require high accuracy adjustment (e.g., automatic dot adjustment value acquisition processing).
Plain paper is a print medium that is inexpensive and relatively easy to obtain. Accordingly, the use of plain paper in printing position adjustment processing sufficiently accommodates the needs of novice users who prefer performing simplified adjustment over high accuracy adjustment.
However, plain paper is a print medium wherein landed ink is likely to bleed among the paper fibers, and has a disadvantage in that variations in the relative positional relationships among reference dots and shift dots in test patterns are poorly reflected on density characteristics or the like. In other words, when printing test patterns using plain paper, it is necessary to vary the relative shift amounts between the reference dots and the shift dots somewhat coarsely to ensure that a predetermined density variation is obtained among adjustment patterns. Therefore, in such a case, since the variation of relative shift amounts of the test patterns is somewhat coarse, an adjustment value having a high adjustment accuracy cannot be acquired, thereby making high accuracy printing position adjustment impossible. As a result, cases will occur where high quality images desired by a user may not be obtained when printing images on high quality print paper such as coated paper on which the influences of dot misalignment are more likely manifested. As shown, there may be cases where the use of plain paper in printing position adjustment makes it impossible to accommodate the needs of users who desire high accuracy printing position adjustment.